Electromechanical servo device



.my 23, 1940. D s. BOND 2,208,623

ELEGTROMECHANICAL SERVO DEVICE -Filed March 5l, 1939 (Ittomeg PatentedAJuly 23, 1940 UNITED STATES PATENT oFFicE Radio Corporation o ofDelaware Application March 31,

9 Claims.

rI'his invention relates to servo devices and more particularly to anapparatus of an electromechanical nature in which one element is movedsynchronously with another element with minimum lag and with freedomfrom instability or hunting It is often desired to indicate the positionof a remotely located movable element by controlling electrically anearby indicator. In devices of this type, some lag is inevitable and,unless suitable precautions are taken, instability and hunting are aptto be present.

One of the objects of this invention is the provision of means forindicating the position of an element by means ofl a second elementactuated by a servo motor. Another object is in the provision of meansfor indicating the position of an element by means of a Vsecond elementmoved in synchronism with the first element. Another object is toprovide means for indicating remotely'the position of an object by meansof la synchronously operated indicator with a mini- ,mum of lag. Alfurther object is to provide means for repeating the movements of anelement by means of an indicator operated synchronously withoutinstability or hunting.

The invention will be described by referring to the accompanying drawingin which Figure 1 is a schematic diagram of one embodiment of theinvention; Figures 2 and 3 are schematic diagrams of the networks andamplifiers used in the invention; and Figures 4a, 4b, 4c and 4d.

illustrate modifications of the network. In describing the severalgures, similar reference numerals will be used to describe similarelements. 4

Referring to Fig. 1, the movable contact l of a potentiometer 3 iscoupled mechanically by a shaft 5 or the like to an element which has amotion gb (t). The fixed terminals of the potentiometer are connected toa double-pole double-throw switch 'l and to the fixed terminals of asecond potentiometer 9. The pairs of the fixed terminals of the switch 1are connected respectively to a D.C. source II and an A.C. source I3.The movable contact of the potentiometer 3 is connected to one of theinput terminals of an electrical network and amplifier I5. The otherinput terminal is connected to the movable contact I1 of the secondpotentiometer 9. The movable contact is coupled mechanically to theshaft of a servo motor I9. The servo motor is connected to the outputterminals of the amplifier I5. l

In the operation of the network, either a D.C. or an A.C. source may beused. If the latter is employed, it is desirable to connect a referencephase from the alternator I3 to the servo motor I9 to determine itsdirection of rotation. o The operation with the D.C. source will flrstbe f America, a corporation 1939, Serial No. 265,265

' described. It will be observed that, for any position of the movablecontact I, a definite input voltage e1 will be applied to the amplifier.This voltage e1 is also Adependent upon the movable contact I1 of thesecond potentiometer 3. The current applied to the servo motor alsodepends upon the input and output voltages of the amplifier.Furthermore, for each position of the movable contact I, a voltage isdeveloped which l The voltages corresponding to such difference passthrough the network and amplifier and actuate the motor which brings themovable contacts into positions in which the input voltages arebalanced. If these movements are to be effected without hunting andwithout too much lag, the electrical and mechanical constants must bechosen as will be hereinafter explained.

' It will be noted that, in the circuit of Fig. 2, the input isrepresented as ei and the output as e2. The output of the firstamplifier Vi includes a resistor R1 and an inductor L1. The secondamplifier V2 has an output circuit including resistor Re and inductorLa. If this circuit is used in the system of Fig. 1, and if the movablecontact l moves with a constant angular velocity w1, the lag angle ai ofthe slidable contact I1 will equal lia where R is the mechanical dampingconstant of the motor, K is a constant and m is proportional to GRiRa,Where G is the amplifier gain. If hunting is to be prevented, theconstants L1,

L2, R1, and R2 must be chosen to satisfy the following equations:

where L is the moment of inertia of the output system and K1, m areconstants.

If the lag angle is to approach zero for an input movement of constantangular velocity w1, the circuit of Fig. 3 may be used. The circuit ofFig. 3 corresponds to Fig. 2 as far as tubes V1,

V2, and V3 are concerned. The difference found in the added tube V4.whose output circuit includes a capacitor C shunted by a resistor Rosatisfying the condition Ro Z. The desired stable condition may berepresented as where is the mechanical damping constant of the motor, K2is'a constant, q1 is m equals G1K1LlL2(Al+ A2) C L is the moment ofinertia of the driven system and m1 equals in which A1 and A2 are thedamping factors of the circuits.

The foregoing circuits are used for direct current sources Il; ifalternating currents are applied I3, two changes are desirable. Thefirst modification is to apply a current of reference phase Ito an A.C.operated servo motor thereby to determine the sense of rotation of themotor rotor. The second modification is shown in Fig. 4b or Fig. 4d. Inthe former figure, a capacitor C1 is connected in series with inductorL1 and the circuit-C1L1 is tuned to resonance. The modlfied circuit'isapplied to tubes V1 and V2 of Fig. 2. The circuit of tube V4 of Fig. 3is modified by shunting an inductor-Lu across the capacitor C. Thecircuit CLo is tuned to resonance. For convenience, the D.C. circuitsFigs. 4a and 4c ,are shown so that they may be compared to Figs. 4b and4d, respectively.

Thus, the invention has been described as a servo or follow-up mechanismwhich is operated by electro-mechanical means. The elecrical Ameansforms a bridge which is balanced when there is no angular difference invposition of the movable contacts of a pair of potentiometers. 'I'hemovable contact of the input or rst potentiometer is connected to -theelement whose movements are to be followed. The movable contact of thesecond potentiometer is connected to a. servo motor which is connectedto the output of the network and amplifier forming part ofthe bridgecircuit. 'I'he network includes` resistive and reactive elements whichrespond to fundamental, and first and second derivative terms expressingmovements of the element whose movements are to be followed. It shouldtbe understood that any desired number of mathematical' terms ordifferentiation circuits may be employed. Likewise the invention is notlimited to following angular movements but may be used to reproduce orfollow other forms of movement.

I 'claim as my invention:

1. In a device of the character described, a pairof potentiometers,means for coupling the movable contact of the first of saidpotentiometers to an element whose movements are to be followed, a servomotor, means coupling the shaft of said motor to the movable contact ofthe second of -said potentiometers, a source of current, means forapplying said current to said potentiometers, means for derivingopposing potentials is from said potentiometers,

an electrical network, said electrical network including a plurality ofvacuum tubes for non-reactively coupling, a resistance and animpedancedevice responsive re-` spectively to the steady state currentscorresponding to the angular position of said rst potentiometer movablecontact and to the variable currents corresponding to the rate of changeoi.' position of said iirst potentiometer movable contact, to a secondresistance and asecond impedance responsive respectively to the currentscorresponding to said angular position and said changing position and tocurrents corresponding to the first derivative of said rate of change ofposition, means for applying said opposing potentiais to the input ofsaid network, and means connecting said motor to the output of saidnetwork.

, 2. In a device of the character described, a pair of potentiometers,means for coupling the movable contact of the first of saidpotentiometers to an element whose movements are to be followed, a servomotor, means coupling the shaft of said motor to the movable contact ofthe.

second of said potentiometers, a source of alternating current, meansfor applying said alternating current to said -potentiometers, means forderiving opposing potentials from said potentiometers, an amplierincluding an electrical network, said amplier including vacuum tubes forcoupling non-reactively a first serially connected resistor and reactorto a second serially connected resistor and reactor so that said firstserially connected resistor and reactor will be responsive respectivelyto the fundamental term and the first derivative of currents applied tosaid amplifier input and said second serially connected resistor andreactor will be responsive respectively to said fundamental and rstderivaing a non-reactive coupling for an inductor and a capacitorresonant to the frequency of said alternating current. a

4. A follow-up mechanism including a movable element, a source ofcurrent, means for deriving from said source two currents, one of saidcurrents being a function of the position of said element, the other ofsaid currents -being a function of the position of an element indicatingthe position of said movable element, an amplifier, said amplifierincluding thermionic tubes for coupling non-reactively seriallyconnected reactive and non-reactive elements to succeeding seriallyconnected reactive and non-reactive elements responsive respectively tothe currents including a fundamental and a first derivative term and tocurrents including said fundamental, said first, and a second derivativeterm, means for applying said derived currents lto the input of saidamplier in opposition, a servo motor connected to the output of saidamplifier, and means coupling the armature of said motor to control theposition of said second element, whereby said motor operates upon saidsecond element until equal opposing currents are applied to said input.

5. A device of the character of claim 4 in which the amplifier includesresistive and reactive ele- Cil ments so related to each other and tothe amplier gain and to the damping of the motor that hunting issubstantially prevented.

' 6. A device of the character of claim 4 in which the amplifierincludes resistive and reactive elements so related to each other and tothe amplifier gain and'to the damping of the motor that hunting issubstantially prevented, and the lag of said second element isminimized.

7. A device of the character of claim 2 in which a current of referencephase is obtained from said alternating current source and applied tosaid servo motor to determine the sense of its direction of rotation'.

8. A follow-up mechanism including a variable element, a source ofcurrent, means for deriving from said source two currents, one ofsaidcurrents being a function of said variable element, the other ofsaid currents being a function of an element corresponding to saidvariable element, an amplifier, said amplifier including thermionictubes for coupling non-reactively serially connected reactive andnon-reactive elements to succeeding serially connected reactive andnonreactive elements responsive respectively to the currents including afundamental and a first derivative term and to currents including saidfundamental, said first, and a second derivative term,

l means for applying said derived currents to the input of saidamplifier in opposition, a servo motor connected to the `output of saidamplifier, and means coupling the armature of said motor to control theposition of said second element, whereby said motor operates upon saidsecond element until equal opposing currents are applied to said inputand the application of currents corresponding to said derivative termsprevents undue hunting of said motor.

9. A follow-up mechanism including an element, a source of currentameansfor varying said current as a function of the movements of said elementso that said variation of current includes components corresponding tothe movements of said element, and an ampliiier for amplifying saidvarying currents, said amplifier including thermionic tubesr forcoupling non-reactively serially connected reactivel and non-reactiveelements to succeeding serially connected reactive and non-reactiveelements responsive respectively to the currents including a fundamentaland a first derivative term and to currents including said fundamental,said first, and a second derivative term.

DONALD S. BOND.

